Translocation of CAM Products in the Plant

A functional difference between CAM and C4-photosynthesis is the immediate source of P-enolpyruvate for carboxylation. In C -photosynthesis, it is largely assumed that PEP is regenerated from the 3-C fragment after malate (or oxalace-tate) utilization. Two possibilities seem apparent (1) pyruvate is converted to P-enolpyruvate by pyruvate, Pi dikinase (Hatch and Slack, 1970) or (2) oxalacetate rather than malate is decarboxylated to form P-enolpyruvate by PEP carboxyki-nase (Black et al, 1973). 

These comparisons can be summarized by stating that CAM is functionally similar to C4 plant photosynthesis except that in CAM the sequential carboxyla-tions are separated temporarily, whereas in C4-photosynthesis they are separated spatially. In favorable environments, certain CAM plants can exercise the C3-photosynthetic option to exploit the environment. 

Although the enzymatic machinery for P-enolpyruvate carboxylation in the nonautotrophic pathway, C3-photosynthesis, C4-photosynthesis, and CAM is similar, the enzymes are sufficiently different to be classed as isozymes, and hence, we assume evolutionary pressures have resulted in a common theme (PEP carboxylation) being modified for similar, but functionally different adaptive roles (see Chap. 4). 

Little is known about translocation in CAM plants. In one study, Chow et al. (1966) exposed Opuntia polyacantha stem sections to C02 in the light for 30 min and assessed the amount of methanol-soluble C-compounds in the exposed 

A similar study of dark by Sutton with Opuntia bigelovii yielded the 

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